The present invention generally relates to optical filters, and more particularly to wavelength-selective optical filters.
The increased use of all-optical fiber networks as backbones for global communication systems has been based in large part on the extremely wide optical transmission bandwidth provided by optical fiber. This has led to an increased demand for the practical utilization of the full optical fiber bandwidth available, to, for example, increase communication system user capacity. In the prevailing manner for exploiting optical fiber bandwidth, wavelength-division multiplexing (WDM) and wavelength-division demultiplexing (WDD) techniques are used to enable the simultaneous transmission of multiple independent optical data streams, each of a distinct wavelength, on a single optical fiber, with wavelength-selective WDM and WDD control provided for coupling of the multiple data streams with the optical fiber on a wavelength-specific basis. With this capability, a single optical fiber can be configured to simultaneously transmit several optical data streams, e.g., ten optical data streams, that each might not exceed, say, 10 Gb/s, but that together represent an aggregate optical fiber transmission bandwidth of more than, say, 100 Gb/s.
In order to increase the aggregate transmission bandwidth of an optical fiber, it is generally preferred that the spacing of simultaneously transmitted optical data streams, or optical data xe2x80x9cchannels,xe2x80x9d be closely packed, to accommodate a larger number of channels. In other words, the difference in wavelength between two adjacent channels is preferably minimized. This desire for closely-spaced optical transmission channels results in the need for fine wavelength resolution and thereby complicates the wavelength-selective WDM and WDD operations required for simultaneous transmission of the channels.
There are a number of optical devices presently available for constructing wavelength-selective WDM and WDD systems. Some of these devices include, for example, thin film filters that reflects a very narrow band of wavelengths. Such filters are often constructed from several hundred layers of stacked narrow band filters, and are designed to reflect a single narrow band of wavelengths. Arrayed waveguide gratings are also available. A limitation of many of these devices is that they are not wavelength tunable. That is, the operative wavelength cannot be dynamically changed during operation in order to select a different optical data channel during use. This can have negative implications for many wavelength-selective WDM, WDD and routing applications.
To overcome these limitations, a number of devices have been developed to provide some level of wavelength tunability. Many of these devices, however, require some form of physical motion or movement to achieve the desired tunability. For example, one such device includes a substrate with a diffraction grating. The diffraction grating is provided in the path of an incoming light beam. To provide wavelength tunability, the diffraction grating is rotated, which causes the incoming light beam to strike the diffraction grating at a different incident angle. The change in incident angle alters the selected wavelength of the grating. In another example, a Fabry-Perot cavity is provided with two mirrors separated by an intervening space. The mirrors are moved either toward or away from each other to vary the intervening space, which changes the selected wavelength of the Fabry-Perot cavity. A limitation of many of these devices is that the required physical movement tends to limit the resolution that can be achieved, and may reduce the reliability and/or stability of such devices. These limitations generally characterize the insufficiency of conventional all-optical wavelength-selective techniques in meeting the increasingly complex requirements of optical systems.
The present invention overcomes many of the disadvantages of the prior art by providing a tunable optical filter that has no moving parts. In one illustrative embodiment, a Fabry-Perot cavity structure is provided that has a top mirror spaced from a bottom mirror, with one or more intervening layers therebetween. The one or more intervening layer preferably has a refractive index that changes with temperature. By heating the one or more intervening layer, the wavelength selected by the Fabry-Perot cavity can be controlled, which provides the desired wavelength tunability or selectivity of the optical filter. The one or more intervening layer is preferably heated by passing a current through the intervening layer, or by passing a current through a separate resistive layer that is thermally coupled to the one or more intervening layer. It has been found that such a filter can provide a high degree of wavelength selectivity in a robust and stable manner.
To reduce the power required to heat the one or more intervening layer, steps may be taken to thermally isolate the one or more intervening layer from its surroundings. In one illustrative embodiment, the one or more intervening layer is mechanically suspended in a cavity by a patterned support layer. The patterned support layer preferably has a low coefficient of thermal conductivity, and may be patterned to have a relatively small lateral cross sectional area. Both of these reduce the amount of heat lost laterally through the device. To reduce the thermal loss due to convection and/or conduction heating, the device may be mounted in a vacuum package, if desired.
A controller is preferably provided for controlling the current that is applied to heat the one or more intervening layer. The controller may be an open loop controller, which provides a specific amount of current or power to select a desired wavelength. Alternatively, the controller may be a closed loop controller, which uses a temperature sensor to provide feedback to the controller so that a desired temperature can be maintained at the layer of intervening material.
A number of methods for making a tunable filter are also contemplated. In one illustrative method, a substrate is first provided. A heater film is then provided adjacent the substrate, such that the heater film is thermally coupled to at least a portion of a predefined filter region of the substrate. A support film is also provided, such that the support film is mechanically coupled to the filter region and to a support region of the substrate. The support region preferably is spaced from and encircles the filter region, but this is not necessary. An upper multi-layer mirror is preferably provided adjacent at least a portion of the filter region. With the upper multi-layer mirror protected preferably using a protective layer, the substrate around the periphery of the filter region is selectively removed, leaving a space between the support region of the substrate and the filter region of the substrate. A lower multi-layer mirror may then be provided below the filter region.
In one illustrative embodiment, a Silicon-On-Insulator (SOI) substrate is used. The SOI substrate has a lower silicon layer, an intermediate insulating layer, and an upper silicon layer. In this embodiment, the filter region is preferably formed in the upper silicon layer, and the lower silicon layer is removed below the filter region. The intermediate insulating layer can be used as an etch stop when removing the lower silicon layer. The intermediate insulating layer can also be removed, or left in position to provide additional support to the filter region if desired. By removing the lower silicon layer below the filter region, the thermal mass of the filter region may be minimized. This reduces the power required to heat the layer of intervening material in the filter region. Once the lower silicon layer is removed, a lower multi-layer mirror can be provided adjacent the upper silicon layer in the filter region. In the above illustrative embodiments, the order of the steps may be changed without deviating from the scope of the present invention.
The tunable filter of the present invention may have a wide range of applications, including telecommunications applications such as WDM, WDD, and routing applications. In one example, the tunable filter of the present invention may be used in a signal drop application where a particular wavelength signal or xe2x80x9cchannelxe2x80x9d is dropped from a multiple channel data stream. In this application, a multiple channel data stream is provided to the tunable filter. The tunable filter is heated such that the tunable filter passes a desired drop signal or channel to a first collector location, and reflects the remaining signals or channels to a second collector location.
To drop another signal or channel, a second tunable filter may be provided at the second collector location. Like above, the second tunable filter may be heated such that the tunable filter passes another desired drop signal or channel to a third collector location. The second tunable filter may reflect the remaining signals or channels to a fourth collector location. This may continue, dropping as many of the signals or channels from the multiple channel data stream as desired.
In a signal add application, where a particular wavelength signal or xe2x80x9cchannelxe2x80x9d is to be added to a multiple channel data stream, the multiple channel data stream may be provided to a first side of the tunable filter. The signal or channel to be added is provided to the opposite side of the tunable filter. The tunable filter is heated such that the tunable filter passes the signal or channel to be added to a first collector location. The tunable filter also reflects the multiple channel data stream to the first collector location.
To add another signal or channel, a second tunable filter may be provided at the first collector location. The signals or channels that are present at the first collector location are provided to a first side of the second tunable filter. Another signal or channel to be added is provided to the opposite side of the second tunable filter. The second tunable filter is heated such that the tunable filter passes the second signal or channel to be added to a second collector location. The second tunable filter also preferably reflects the signals provided by the first tunable filter to the second collector location. This may continue to add as many signals or channels to the multiple channel data stream as desired.
Another illustrative application for the tunable filter of the present invention is to monitor the emission wavelength of a laser. It is known that the emission wavelength of a laser may drift over time, temperature, etc. To monitor the emission wavelength, the tunable filter of the present invention may be positioned between the laser and a detector. The detector preferably is capable of detecting a relatively wide range of wavelengths, while the tunable filter only passes a relatively narrow band of wavelengths. With the laser turned on, the tunable filter is heated until the filter passes the current operating wavelength of the laser to the detector. When the detector detects the emission, a controller is notified. By noting the heat applied to the tunable filter, the controller may determine the current operating wavelength of the laser.
Such a system may be used to, for example, control the wavelength of a laser. For example, if the current operating wavelength of the laser, as determined by the controller, is not within a predefined range of wavelengths, the controller may adjust the power that is applied to the laser to change the emission wavelength of the laser until it falls within the predefined range of wavelengths. It is contemplated that the laser emission may be directly applied to the tunable filter. Alternatively, only a portion of the laser emission may be applied to the tunable filter by using a beam splitter or the like.